It was 5 days ago when it was given away on IndieGala (and it is actually free on there again now):
http://www.steamgifts.com/discussion/DQepS/grab-cubetractor-for-free-indiegala
Either you got it from there or you are re-gifting from another site, which isn't actually a problem as long as it's not against their rules. You may want to check as it's obvious you haven't read the rules on SteamGifts.

Despite that, if the game is not listed it is not valid as a give-away and you can not give-away a different game than specified.
The reason for this is that you will points for the game that you specified (5 CV for Polarity) yet the game you will give away is worth a different amount of points (0 CV for an unlisted game). If this was allowed, then everyone would be posting copies of GTA 5 and in the comments saying it was actually for a free key.

The winner of this give-away will also not be able to accept that they received the key as eventually it will be noticed that they have accepted a key for Polarity but do not have it in their library. This indicates that they 'may' have re-gifted the game and as such they will get suspended from the site.

To tell you the truth Polarity is only 15p at the moment, so you would be best of buying a copy to give away and modify your comment to explain and apologise as if you cancel the give-away now you will lose a give-away slot.

Hopefully you will do the right thing and learn from the mistake.
Good luck with future give-aways.

Edit: Oh! Just noticed it had already ended.
You may still want to purchase Polarity and give it to the winner to save the give-away slot and the possibility of a suspension.